Generally, a polymer electrolyte membrane fuel cell (PEMFC), which is a device generating electricity through an electrochemical reaction between hydrogen and oxygen, has efficiency higher than that of other types of fuel cells, has a current density and an output density larger than those of other types of fuel cells, has an ignition time shorter than that of other types of fuel cells, and uses a solid electrolyte, such that the electrolyte does not need to be managed. In addition, since a reaction product of the polymer electrolyte membrane fuel cell is pure water, the polymer electrolyte membrane fuel cell is an environmentally-friendly power source. Therefore, a study on the polymer electrolyte membrane fuel cell has been actively conducted in an automobile industry.
A fuel cell is a cell individually configured in order to obtain large power and having a voltage of 0.6 to 1V, and electrically large energy may be obtained by stacking several cells. A product in which individual cells are stacked one by one is called a fuel cell stack. Generally, three hundred or more cells may be stacked to drive a fuel cell vehicle.
Here, the cell in the fuel cell stack is configured to include a membrane-electrode assembly becoming a path of an electrochemical reaction and hydrogen ions, a separator moving a reaction gas and electrons, a gas diffusion layer uniformly diffusing the reaction gas to electrodes, and a gasket isolating hydrogen used as the reaction gas at the time of stacking the cells and a coolant used in order to cool air from each other and preventing the hydrogen and the coolant from being leaked to the outside. The membrane-electrode assembly is again divided into a membrane and the electrodes. A perfluorosulfonic acid based solid polymer is mainly used as a material of an electrolyte membrane used in the membrane-electrode assembly, and a thin membrane having a thickness of 10 to 30 μm is generally used in order to lower ion conductivity having a large influence on performance of the fuel cell.
In addition, the polymer electrolyte membrane fuel cell is generally driven at a temperature of −30 to 80° C. due to characteristics of a polymer membrane used in the polymer electrolyte membrane fuel cell. The polymer membrane needs to secure conductivity in order to implement high performance. A content of water has the largest influence on the conductivity. Therefore, humidification is made using a humidifier or through recirculation, in addition to water electrochemically generated in the fuel cell itself at the time of driving the fuel cell, and water management becomes an important driving strategy in order to constantly maintain water remaining in the fuel cell. However, an excessive amount of water present in the fuel cell has a negative influence on performance and endurance of the fuel cell at the time of driving the fuel cell. This is called flooding. In terms of performance, an excessive amount of water hinders the reaction gas from arriving at the electrodes to significantly increase a material transfer resistance, thereby decreasing performance and causing a cell voltage fluctuation. As a result, carbon corrosion of a cathode electrode as well as local carbon corrosion of an anode electrode may be caused, which may have a large influence on performance the fuel cell vehicle. Therefore, a countermeasure for preventing the problem described above is required.
To this end, a fuel cell system according to the related art may be a system in which a humidifier is mounted, as illustrated in FIG. 1. Water generated through an electrochemical reaction is utilized for humidification of a reaction gas through the humidifier, thereby making it possible to raise conductivity of an electrolyte membrane within a fuel cell stack. As a result, outputs of the fuel cell stack and the fuel cell system may be increased.
However, in the case in which the humidifier is mounted in the fuel cell system as described above, a structure of the fuel cell system becomes large due to a volume of the humidifier.
A non-humidified system in which an external humidifier is removed as illustrated in FIG. 2 has been developed in order to solve the problem described above. However, the non-humidified system may not more smoothly supply water as compared with the system in which the humidifier is mounted, such that efficiency is decreased.